The present invention relates to thin sheet foamed products, especially radiation cross-linked thin sheet olefin polymer or copolymer foamed products. Thin sheet foamed products are typically products having a thickness in the range from 1/16 to 1/4 inch, and which undergo expansion to a final density of approximately two pounds per cubic foot (pcf).
Foam sheets having a thickness greater than 1/4 of an inch are typically expanded in a horizontal oven, generally over a cushion of air, which supports the foam in the oven as it expands Unfortunately, such horizontal hot air, floating foam expansion ovens do not operate satisfactorily in the expansion of thinner sheets, that is, sheets of less than about 1/4 inch thickness. The thinner sheets tend to fold over on themselves and wrinkle as they are expanded in horizontal ovens.
One particular type of foam which has to be expanded in relatively thin films is radiation cross-linked polyolefin or polyolefin copolymer foam. Radiation is an effective cross linker only up to thicknesses of about 1/4 of an inch. Radiation cross-linked foams are particularly desired in the health care industry, primarily for aesthetic reasons; their fine cells, smooth surface and soft feel make them superior to chemically cross-linked foam.
To overcome wrinkling and folding problems associated with thin sheet expansion in a horizontally oriented hot air foam expansion oven, cross-linked thin sheets have been expanded in a vertically oriented tentering frame. Such vertical expansion devices are disclosed in U.S. Pat. Nos. 4,124,344 and 4,231,925. These vertical expansion devices have the advantage of eliminating fold-over and wrinkles in the thin sheet foam product.
Cross-linked thin sheet foam products tend to be oriented non-uniformly across their width. Orientation, in turn, negatively affects thermoforming of the film into a desired end product, since the foam, when heated during the thermoforming, releases stresses which were induced during the foam's manufacture All foams possess this orientation to a greater or lesser degree; the problem arises from inconsistent orientation. This is especially noticeable where a wide roll of film, for example, 60 inches wide, may be slit into four to six separate rolls. The change in orientation across the original 60 inch sheet results in the separate rolls having decidedly different thermoforming characteristics because of the non-uniform orientation across the width of the foam sheet.